A global system for mobile communication (GSM) is one of wireless technologies which have been developed to accommodate a large number of subscribers ever since it was created as one of means for unifying wireless communication mechanisms in the European region. A general packet radio service (GPRS) is introduced to provide a packet-switched data service in a circuit-switched data service provided in the GSM. An enhanced data rate for GSM evolution (EDGE) provides a higher data capacity by using an additional modulation scheme in the GPRS.
In a wireless communication system, a mobile station communicates with a base station. When a link between the base station and the mobile station is disconnected, this is called a radio link failure. The purpose of determining the radio link failure by the mobile station is to re-establish or release a cell with unacceptable voice/data quality.
According to the section 5.2 of 3GPP TS 45.008 V7.9.0 (2007-08) “Technical Specification Group GSM/EDGE Radio Access Network; Radio subsystem link control (Release 7)”, a criterion for determining the radio link failure in the mobile station shall be based on a decoding success rate of messages on a downlink slow associated control channel (SACCH).
A traffic channel (TCH) is used for transmission of voice or circuit switched data. Signaling related to the TCH is transmitted on the SACCH or a fast associated control channel (FACCH).
In the conventional GSM/GPRS system, a coding rate of the SACCH is higher than a coding rate of the TCH. Sensitivity to a channel state increases in proportion to the coding rate. Therefore, if the decoding of the message on the SACCH fails, a decoding failure rate of the TCH is high.
However, as an adaptive multi-rate (AMR) audio codec is applied, the coding rate of the TCH becomes variable, and there is a case where the coding rate of the SACCH becomes lower than the coding rate of the TCH. This implies that the decoding of the SACCH fails but a decoding success rate of the TCH increases.
In this state, when the radio link failure is determined only based on a decoding success rate of the SACCH, there may be a problem in which the radio link failure is detected even if voice/data service can normally be provided.